The quality of paper is often judged by its brightness, opacity, and sizing (or water repellency). Paper producers have long sought to improve these vital characteristics so that an enhanced paper may be obtained.
These three desired characteristics have been obtained in the past by supplying the slurry or furnish with additives prior to the slurry entering the papermaking machine. Various additives are well known in the art. For example, titanium dioxide powder is known to be an excellent whitener. Titanium dioxide, however, is among the most expensive materials that may be added to the slurry. Thus, despite the effectiveness of such material as a brightener, its use is limited and satisfactory replacements have been needed.
Kaolin clay has also been used as a filler in paper to improve brightness in the ultimate product. Generally, the kaolin clay is calcined and then suspended in an aqueous solution prior to being added to the furnish. The clay must be continuously agitated prior to entering the slurry or the solid particles begin to form sediment at the bottoms of the clay holding tanks. Although kaolin clay provides brightness, as well as opacity to the finished paper product, the relative difficulty of adding it to the slurry results in a less than excellent additive.
Furthermore, when clay is added to the slurry, the slurry becomes thicker, thus resulting in a slurry having a higher coefficient of friction. The papermaking process using a kaolin-containing slurry therein increases the processing time relative to the time required for processing a pulp slurry not containing kaolin. Moreover, because the kaolin particles are solids and never completely dissolve in the aqueous solutions, the clay tends to clog or foul the mesh of the wire or fabric on the Fourdrinier or other papermaking machine, thereby resulting in a large amount of down time for cleaning the wire or fabric of the machine. Such kaolin-containing products are described in U.S. Pat. Nos. 3,014,836 to Proctor, Jr. and 4,826,536 to Raythatha et al.
Hydrated aluminum silicate has also been employed as a clay substitute in the papermaking process. It has properties similar to kaolin clay and, thus, results in the same disadvantages when used to make paper.
Many compositions have been added to the slurry in an attempt to size the paper, i.e., add body to the paper and render the paper water repellent or waterproof. Most known sizes, such as those disclosed in U.S. Pat. No. 2,142,986 to Arnold, Jr. and U.S. Pat. No. 3,096,232 to Chapman, employ a type of wax. For example, Arnold, Jr. discloses that an emulsion of wax in a solution of deacetylated chitin, paraffin waxes, Japan wax, carnauba wax, higher aliphatic alcohols, or synthetic waxes may be employed as the waterproofing agent in a sizing composition. A softening agent such as aliphatic alcohols containing 12 to 20 carbons is also present in the composition of Arnold, Jr. Chapman discloses the use of paraffin waxes or water-insoluble derivatives of resins for producing aqueous wax emulsions with cationic modified starches.
U.S. Pat. No. 2,772,967 to Padbury shows a paper sized by adding thereto a salt of a high molecular weight composition prepared by reacting a dialkanolamine or trialkanolamine with a long chain fatty acid. The salt is diluted with water to form a dispersion containing a 5% concentration of sizing agent before being applied to the cellulosic fibers. Apparently, such a dilution of strength was necessary heretofore because until the present invention, preparation of the stearamides which would allow the composition to remain pourable at concentrations greater than 5% was unknown. Without the ability to remain in an emulsion and, hence, be poured, concentrations of stearamides approaching those disclosed herein have not been possible for use on pulp fibers. An important feature also disclosed by the patent is that the salts are cationic and are, therefore, adsorbed by the anionic cellulosic fibers.
Numerous sizing agents are known. Generally, the known sizes are cationic materials, particularly those used to size fabrics for the textile industry. Although the sizes' cationic nature increases their absorption by the fibers to which they are applied, their cationic nature generally prevents them from being used to the full extent possible in connection with a brightener and opacifying agent. It is well known in the art that although cationic materials often increase sizing, they reduce the brightness of the material to which they are applied. Although this is not generally a problem in the textile industry where sizing is important but opaqueness and brightness may be sacrificed, the use of cationic sizes in the paper industry reduces the quality of the paper made therefrom. Because the addition of cationic sizing agents to paper generally reduces the brightness thereof, cationic sizes have not been heretofore preferred as a size for paper, and in particular, as a size for paper made from recycled pulp which often lacks the inherent brightness of paper made from virgin pulp.
Although the prior art shows agents for sizing paper and agents for increasing the brightness and opaqueness of paper, the particular features of the present invention are absent from prior art. The prior art is generally deficient in affording a composition for use in a papermaking process that has the ability to provide sizing to paper without reducing brightness or opacity. Furthermore, the prior art brighteners and opacifying agents fail to allow continuous running of papermaking machines due to the fouling tendency of the forming fabrics. The present invention, however, overcomes the shortcomings of the prior art in that a composition is disclosed herein for simultaneously increasing the brightness, opacity, and sizing of paper made from a slurry containing the composition.